1. Field of the Invention
The present invention relates to a semiconductor device improved such that first and second insulating films each having a reflectance which is 25% or more and periodically changes in accordance with a change in film thickness are combined with each other and formed on a conductive layer formed on a semiconductor substrate to obtain a total reflectance of less than 25% of the first and second insulating films, and a method of manufacturing the same.
2. Description of the Related Art
In recent years, the integration density of a semiconductor device has increased considerably, and, in accordance with this, not only the degree of micropatterning of a circuit has increased, but also the degree of micropatterning and the number of layers constituting a wiring multilayer have increased. For this reason, in a process of forming a pattern for a gate electrode of a wiring layer, the thickness and size of the pattern must be strictly controlled. In particular, in a photoetching process, although a mask pattern must be faithfully formed on the surface of a semiconductor substrate, when the degree of micropatterning increases as described above, pattern formation is adversely affected when light from a light source is slightly reflected in an exposing process. More specifically, when a conductive film such as a metal material on which a pattern is to be formed has a high reflectance with respect to the wavelength (to be referred to as an exposure wavelength hereinafter) of light from a light source for performing exposure, a more serious problem is posed. As the material of the mask pattern, a photosensitive film such as a photoresist film is used. Although this photoresist is exposed along a desired mask pattern by light directly incident on the surface of the photoresist, the photoresist is actually exposed by light reflected by the pattern formation conductive layer. For this reason, when the pattern formation conductive layer has an uneven surface, the photoresist is exposed to have a shape different from that of the desired mask pattern because the optical path of light reflected by the uneven surface does not coincide with the optical path of the incident light.
As is apparent from the sectional view showing a step in manufacturing a conventional semiconductor device shown in FIG. 1, a metal layer 1 serving as a pattern formation conductive film is formed on a semiconductor substrate 10 consisting of silicon or the like through an insulating film 12, and a photoresist 2 is formed on the metal layer 1. A polysilicon electrode 11 is formed on the semiconductor substrate 10 through a gate oxide film 13 adjacent to the photoresist 2, and the metal layer 1 is formed on the polysilicon electrode 11 through the insulating film 12.
When light from a light source is radiated on the photoresist 2 through a reticle or photomask (not shown), the light is radiated on the photoresist 2 at an almost right angle. The irradiated portion is dissolved in a developing solution, and the photoresist portion not irradiated with light is not dissolved to be left as a mask pattern. This photoresist is called a positive type resist. A negative type resist in which an exposed portion is not dissolved in a developing solution may also be used. In any case, a material to be patterned is etched using the photoresist left on the semiconductor substrate as a mask. Therefore, the mask pattern must be formed to faithfully reproduce the reticle.
When light used for exposure is reflected by the underlying metal layer 1, the light is reflected in the incident direction as indicated by an arrow in a region A where the polysilicon film 1 has an even surface. However, in a region B having a portion in which the gate electrode 11 or the like is formed to form a stepped portion 3, the light is not reflected in the incident direction, and the reflected light propagates in an upper right direction (the direction of arrows shown in FIG. 1) and is radiated on a side surface portion of the photoresist 2. As a result, the side surface of the photoresist 2 left as the mask pattern is partially exposed to form a recessed portion 4. Due to the recessed portion 4, even when etching is performed using the developed photoresist 2 as a mask pattern to form a wiring layer by the underlying metal layer 1, a desired wiring pattern cannot be obtained, and disconnection of the wiring layer may occur in the worst case. In this manner, an accurate developing process for the photoresist cannot be easily performed near the uneven portion of the metal layer, and the above method cannot easily cope with recent semiconductor devices having a higher degree of micropatterning and a larger number of layers constituting a multilayer.
As one method of solving the above problem, the following method as disclosed in U.S. Pat. No. 4,910,122 is known. That is, an anti-reflection film for decreasing the reflection amount of the conductive layer is deposited on the pattern formation conductive layer 1, and the photoresist 2 is coated on the anti-reflection film. For example, FIG. 2 is a sectional view showing a wiring portion on the semiconductor substrate for explaining an example of the above method. An anti-reflection film 5 which can be dissolved in an alkaline solution is spin-coated on the polysilicon film 1 formed on the semiconductor substrate 10 through the insulating film 12, an annealing process is performed for the resultant structure, and the photoresist 2 is coated on the anti-reflection film 5. This photoresist 2 is exposed and then developed in an aqueous alkaline solution. However, according to this method, since not only the photoresist 2 but also the anti-reflection film 5 serves as a mask together with the metal layer 1 serving as the pattern formation conductive layer, its processing accuracy is determined by the pattern of the anti-reflection film 5. However, the pattern size of the anti-reflection film 5 is not easily controlled because not only a dissolution amount of the anti-reflection film 5 must be controlled but also a change in thickness of the anti-reflection film 5 must be controlled. In addition, a method of inserting a metal layer or a metal compound layer for decreasing a reflectance between the metal layer 1 serving as the pattern formation conductive layer and the photoresist 2 is known in Jpn. Pat. Appln, KOKAI Publication No. 60-240127. However, according to this method, the following various problems are posed to avoid practical use of the semiconductor device. That is, film quality is made unstable after a pattern is formed, and a trouble occurs a subsequent process of performing a high-temperature process, and a special process of removing the metal layer or metal compound layer must be performed.
In addition, as described in U.S. Pat. No. 3,884,698, it is known that a conventional anti-reflection film has a single layer. In this anti-reflection film having the single layer, the minimum value of a reflectance which periodically changes in accordance with a change in film thickness is determined for each predetermined film thickness. For this reason, the reflectance is not easily decreased to 25% or less, and the reflectance is not easily changed by controlling the film thickness.